The straphanger stabbed to death on a Brooklyn train had been harassing a couple on board – and punched the woman before the man knifed him in the chest, police sources said Wednesday. 

The 36-year-old bully was hassling other commuters on board a northbound J train approaching Marcy Avenue in Williamsburg Tuesday evening before he approached the pair and started arguing with them, the sources said. 

During the clash, he socked the woman in the face, the sources said. 

Her partner, 20, then knifed the menace in the chest, according to the sources. 

The wounded man stumbled out of the train and onto the platform, the sources said. 

Officers responded to a 911 call about the stabbing just after 8 p.m., cops said.

EMS also responded and rushed the 36-year-old to Brooklyn Methodist Hospital, where he was pronounced dead. 

The couple stayed on the train, but were tracked down by police at the Chauncey Street station, the sources said. 

Police have said the three do not know each other.

Both were initially taken into custody as “persons of interest,” but the woman — whose age is unclear — was later released, police confirmed Wednesday morning. 

The man was not immediately charged, pending the NYPD’s conferral with the Brooklyn DA’s Office, cops said. 

The stabbing victim had not been identified by late morning, pending family notification.

The incident comes about two months after Marine Daniel Penny, 24, placed Jordan Neely, 30, in a deadly chokehold on board an F train in Lower Manhattan — when the older man verbally lashed out at other commuters and threw trash around the car.

Penny was charged with manslaughter and is free on $100,000 bail.

Penny said in a recent interview that he believes Neely — who suffered from mental illness — “would have killed somebody” if he had acted on the threats he yelled at straphangers.